Estimation of radio channel bit error rate in a digital radio telecommunications network

ABSTRACT

A system for determining when a radio telecommunications network should switch between a first grade of service and a second grade of service based upon a highly accurate and reliable estimate of the bit error rate (BER). The system estimates the BER on an uplink and a downlink on a user channel, and determines whether the BER on the uplink and the downlink on the user channel are below a first set of thresholds. The system then measures the residual BER (RBER) and the frame erasure rate (FER) on the voice channel uplink, and determines whether the downlink BER, the uplink RBER, and the uplink FER are lower than a second set of thresholds. If lower, the system sends and receives a data message on a Fast Associated Control Channel (FACCH). The system obtains a highly accurate estimate of the BER on a downlink on the FACCH channel, and reports the downlink BER in a message on the FACCH channel uplink. The system then estimates the BER on the FACCH channel uplink. Finally, the system switches the radio telecommunications network from the first grade of service to the second grade of service in response to determining that the uplink and downlink BER on the FACCH channel are below a third set of thresholds.

This application is a divisional of patent application Ser. No.08/846,630 which is now as U.S. Pat. No. 5,828,672, filed on Apr. 30,1997.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

This invention relates to radio telecommunication systems and, moreparticularly, to a system and method of estimating radio channel biterror rate in a digital radio telecommunications network.

2. Description of Related Art

In digital radio telecommunications networks, the quality of thetransmitted signal is often expressed in terms of how many of thereceived bits are corrupted, leading to the expression Bit Error Rate(BER). The BER indicates how many of the total number of bits arewrongly detected.

With the introduction of data services in radio telecommunicationsnetworks, systems that provide two grades of service are beingdeveloped. A high grade of service utilizes high level modulation toprovide higher service quality for data or improved voice services. Alower grade of service utilizes low level modulation to provide servicequality which is adequate for traditional voice applications. A systemand method of switching between one grade of service and the other isneeded. Such a system must identify which level of service is beingprovided, assess the adequacy of the provided service, and if theservice is not adequate, perform a switch between the two. In order toaccomplish these objectives, a more accurate method of determining theBER is needed.

Although there are no known prior art teachings of a solution to theaforementioned deficiency and shortcoming such as that disclosed herein,U.K. Patent Application Number GB 2,232,854 A (GEC-Marconi), U.S. Pat.No. 5,418,789 (Gersbach), and U.S. Pat. No. 5,406,562 (Roney) discusssubject matter that bears some relation to matters discussed herein. TheGEC-Marconi patent discloses a method of assessing the quality of a datachannel based on the processing of soft decision demodulationinformation. The BER may be determined more quickly because samplingtime is reduced due to the use of soft decisions within a known band oflevels rather than hard decisions which require longer sampling time.The GEC-Marconi patent, however, does not teach or suggest a system ormethod of determining the BER with such accuracy and reliability that itcan be used to determine when a radio telecommunications network shouldswitch from a low grade of service to a high grade of service.

The Gersbach patent discloses a system and method for rapidly estimatingthe bit error rate of a data signal which has been reconstructed from areceived data signal. A bit error rate calculator is integrated with anearly instantaneous bit error rate estimator which utilizes timing andamplitude degradation information. The Gersbach patent, however, doesnot teach or suggest a system or method of determining the BER with suchaccuracy and reliability that it can be used to determine when a radiotelecommunications network should switch from a low grade of service toa high grade of service.

The Roney patent discloses a BER estimation process which receivesencoded data over a channel, decodes the data, and estimates the numberof errors induced by the channel. Roney discloses a method of estimatingthe BER on a Fast Associated Control Channel (FACCH) as well as the BERon the user channel. The two BER estimates are compared, and thedifference is utilized to determine whether the received data isconvolutionally encoded user information or a FACCH message. The Roneypatent, however, does not teach or suggest a system or method ofdetermining when a radio telecommunications network should switchbetween one grade of service and another based upon a highly accurateand reliable estimate of the BER.

Review of each of the foregoing references reveals no disclosure orsuggestion of a system or method such as that described and claimedherein.

In order to overcome the disadvantage of existing solutions, it would beadvantageous to have a system and method of determining when a radiotelecommunications network should switch between one grade of serviceand another based upon a highly accurate and reliable estimate of theBER. The present invention provides such a system and method.

SUMMARY OF THE INVENTION

In one aspect, the present invention is a system for determining when aradio telecommunications network should switch between a first grade ofservice and a second grade of service based upon a highly accurate andreliable estimate of the bit error rate (BER). The system includes meansfor estimating the BER on an uplink and a downlink on a user channel,and means for determining whether the BER on the uplink and the downlinkon the user channel are below a first threshold. The system alsoincludes means for sending and receiving a data message on a FastAssociated Control Channel (FACCH) in response to determining that theBER on the uplink and the downlink on the user channel are below thefirst threshold. The system also includes means for estimating the BERon an uplink and a downlink on the FACCH channel, means for estimating aresidual bit error rate (RBER) on the uplink on the FACCH channel, andmeans for estimating a frame erasure rate (FER) on the uplink on theFACCH channel. The system then determines whether the BER on thedownlink on the FACCH channel is below a second threshold, whether theRBER on the uplink is below a third threshold, and whether the FER onthe uplink is equal to zero. If all three conditions are met, the systemswitches the radio telecommunications network from the first grade ofservice to the second grade of service.

In another aspect, the present invention is a method of determining whena radio telecommunications network should switch between a first gradeof service and a second grade of service based upon a highly accurateand reliable estimate of the bit error rate (BER). The method begins byestimating the BER on an uplink and a downlink on a user channel, anddetermining whether the BER on the uplink and the downlink on the userchannel are below a first threshold. The method then sends and receivesa data message on a Fast Associated Control Channel (FACCH) in responseto determining that the BER on the uplink and the downlink on the userchannel are below the first threshold. This is followed by estimatingthe BER on an uplink and a downlink on the FACCH channel, estimating aresidual bit error rate (RBER) on an uplink on the FACCH channel, andestimating a frame erasure rate (PER) on the uplink on the FACCHchannel. The method then determines whether the BER on the downlink onthe FACCH channel is below a second threshold, whether the RBER on theuplink is below a third threshold, and whether the FER on the uplink isequal to zero. This is followed by switching the radiotelecommunications network from said first grade of service to saidsecond grade of service in response to determining that all threeconditions are met.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood and its numerous objects andadvantages will become more apparent to those skilled in the art byreference to the following drawing, in conjunction with the accompanyingspecification, in which:

FIG. 1 (Prior Art) is a simplified functional block diagram illustratingthe functions performed in an existing method of estimating the biterror rate (BER) in a radio telecommunications network;

FIG. 2 is a graph of voice quality (VQ) and BER as a function of thecarrier to interference (C/I) ratio for a high level modulation (HLM)scheme and a low level modulation (LLM) scheme in the preferredembodiment of the present invention;

FIG. 3 (Prior Art) is an illustrative drawing illustrating the number ofbits and classes of bits encoded in a speech transmission and in a FastAssociated Control Channel (FACCH) message transmission;

FIG. 4 is a functional block diagram illustrating the functionsperformed in the method of the present invention when estimating the BERfor a speech transmission on the user channel;

FIG. 5 is a functional block diagram illustrating the functionsperformed in the method of the present invention when estimating the BERduring transmission of a FACCH message on the FACCH channel; and

FIGS. 6A-6C are a flow chart illustrating the steps of the preferredembodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

When a radio telecommunications network is operating in the high gradeof service, and it is detected that the radio channel quality isdegrading, then the network must switch back to the lower grade ofservice to maintain voice quality. To detect that voice quality isdegrading with the speech coder on the high rate is relatively simple.When a high bit error rate (BER) is measured, and it exceeds apredetermined threshold, then the system switches the network back tothe lower grade of service.

Changing from the lower grade of service to the higher grade is morecomplicated. The network may be operating in a mode that is lessdemanding in terms of radio channel conditions, and the network,therefore, is already offering a good radio channel quality (i.e., lowBER). The system must decide at that time whether the radio channelconditions are good enough to support the higher grade of service inwhich there is a higher level modulation, higher rate voice coder, etc.Thus, the system must assess the quality of a good channel which has alow BER. Therefore, a very accurate BER measurement is required.

There are advantages in changing to the higher grade of service. Thehigh rate modulation supports the use of a high rate voice coder. Thehigh rate voice coder inherently is better quality than a low rate voicecoder, and provides excellent service. However, if the channelconditions degrade, the voice quality degrades very rapidly whenutilizing the high rate voice coder. In those cases, the system changesto low level modulation with low rate voice coder which is more robust.

Today's techniques for measuring BER are not sufficient to identify thelow BER threshold that triggers the system to change to high gradeservice. FIG. 1 is a simplified functional block diagram illustratingthe functions performed in an existing method of estimating the BER in aradio telecommunications network 1. A transmitter 2 includes an encoder3 which takes bits of data (bit_(data)) and adds redundancy bits to forma transmitted bit stream (bit_(transmit)). As the encoded bit_(transmit)is transmitted over the user channel of a digital traffic channel (DTC)4, errors are incurred. A receiver 5 includes a detector 6, a decoder 7,and an encoder 8 which is identical to the transmitter encoder 3. Thedetector 6 detects the received bit stream (bit_(receive)) whichincludes both correct and incorrect bits.

The bit_(receive) is decoded by the decoder 7 in a process in whicherror correction is performed, and the redundancy bits are removed, toproduce a corrected bit stream (bit_(corrected)). The bit_(corrected) isapproximately the same as the bit_(data), with the exception of residualerrors that could not be recovered. If it is assumed that thebit_(corrected) is equal to the bit_(data), and the bit_(corrected) areapplied through the encoder 8 (which is identical to the transmitterencoder 3), then the output (bit_(encoded)) should be equal tobit_(transmit). However, because of the residual errors in thebit_(corrected), they are not exactly equal. The bit_(encoded) is thensent to a BER estimator 9 where bit_(encoded) is compared tobit_(receive) to determine the BER. Thus, an estimate of the bitstransmitted (i.e., bit_(encoded)) is compared with the bits received todirectly determine BER. However, this calculation is inherently flawedbecause the residual errors in the bit_(corrected) make the estimate ofbit_(transmit) inaccurate.

When the system identifies the BER as being low, this estimate must beaccurate or else changing to the high modulation and high voice coderrate may result in a loss of voice quality rather than a gain.Therefore, it is critical that the system and method for determining theBER be accurate and reliable. The present invention provides a systemand method by which very low BERs are accurately evaluated with a highlevel of confidence.

FIG. 2 is a graph of voice quality (VQ) and bit error rate (BER) as afunction of the carrier to interference (C/I) ratio for a high levelmodulation (HLM) scheme and a low level modulation (LLM) scheme in thepreferred embodiment of the present invention. The C/I is directlyrelated to the radio channel condition. That is, a high C/I indicatesgood radio channel conditions while a low C/I indicates poor radiochannel conditions. The solid line 11 represents BER as a function ofC/I for the low level modulation (LLM) scheme utilizing a 8 kb/s voicecoder (vocoder). The dashed line 12 represents BER as a function of C/Ifor the high level modulation (HLM) scheme utilizing a 16 kb/s vocoder.The solid line 13 represents VQ as a function of C/I for the low levelmodulation (LLM) scheme utilizing a 8 kb/s voice coder (vocoder). Thedashed line 14 represents VQ as a function of C/I for the high levelmodulation (HLM) scheme utilizing a 16 kb/s vocoder.

In very good channel conditions (i.e., high C/I), the voice quality forthe high level modulation scheme is significantly better than the voicequality with the low level modulation. In poor channel conditions (i.e.,low C/I), on the other hand, the voice quality for the low levelmodulation scheme is significantly better than the voice quality withthe high level modulation. There is a crossover point at 15. For systemmanagement purposes, it is desirable to determine when the networkshould change to the high level modulation because the C/I is to theright of the crossover point 15, and when the network should change tothe low level modulation because the C/I is to the left of the crossoverpoint 15. Direct measurements of the C/I are difficult and may beunreliable. Therefore, this determination is made by measuring the biterror rate (BER).

The two curves 11 and 12 show the BER as a function of C/I. For anygiven C/I, the BER is higher for the high level modulation than it isfor the low level modulation. The present invention measures BER. If thenetwork is operating in low level modulation, and the BER falls below athreshold value (BER_(up)) 16, the system changes the network to thehigh level modulation. By changing to high level modulation, voicequality rises from point 17 to point 18 on the graph. BER also rises(from point 19 to point 21), but this rise is acceptable since voicequality is better even at the higher BER.

If the quality of the channel degrades (i.e., the C/I falls), the BERfollows dashed line 12 to the left from point 21. When the BER risesabove a threshold value (BER_(down)) 22, the system changes the networkto the low level modulation. By changing to low level modulation, voicequality rises from point 23 to point 24 on the graph. At the same time,BER falls from point 25 to point 26.

Estimating the BER at BER_(down) 22 is a relatively easy task because itis a high BER. The low BER at BER_(up) 16 is difficult to measurebecause the system has to measure a large number of bits for eachmeasured error. Existing procedures to measure BER are not capable ofproviding the degree of accuracy and reliability required for measuringBER at this level. The BER curve 11 is very flat at this point, andsmall errors in the BER measurement result in large differences in C/Iestimates. These differences can impact greatly the decision to changefrom the low level modulation to the high level modulation.

The present invention evaluates the BER on a Fast Associated ControlChannel (FACCH) rather than the user channel of the DTC. The DTC carriesboth voice bits and data bits, but not simultaneously. The voice bitsare on the user channel and the data bits are on the FACCH channel. Eachslot contains 260 bits. When voice bits are sent in the slot, the bitsare encoded in a particular way. When data bits are sent (for example,when sending a FACCH message), these 260 bits are encoded in a differentway. This encoding is described in more detail below. The encoding fordata bits is more robust than the encoding for voice bits, therefore,error correction is more effective. Since error correction is moreeffective, the bit_(encoded) (see FIG. 1) is a more accurate estimate ofthe bits transmitted. Therefore, when the bit_(encoded) is compared tobit_(receive) to determine the BER, the determination is more accurateon the FACCH channel.

FIG. 3 is an illustrative drawing illustrating the number of bits andclasses of bits encoded in a speech (voice) transmission frame 31 and ina FACCH message (data) transmission frame 32. For the speech frame,there are two classes of voice bits: Class 1 bits and Class 2 bits.There are 77 Class 1 bits. Twelve (12) Class 1a bits 33 have thecyclical redundancy check (CRC) error detection scheme applied to them,and 65 class 1b bits 34 do not. The 77 Class 1 bits (and 7 CRC bits) areprotected with a one-to-two (1/2) coder (i.e., one redundancy bit isadded for each voice bit). Thus, a total of 178 encoded Class 1 bits aretransmitted. There are 82 Class 2 bits 35 which are not encoded (i.e.,they are transmitted with no error protection at all). Thus, a total of260 bits are transmitted.

The payload 36 of the FACCH message frame 32 is 49 bits in length. A16-bit CRC 37 is added to the payload to increase the bits to 65. All 65bits are encoded by a one-to-four (1/4) coder because for every bit thatgoes into the coder, four bits come out of it. The 65 bits multiplied by4 equals 260 bits transmitted over the air. The CRC is an errordetection technique while the encoding is error correction coding. Thus,the signal has a much greater amount of redundancy to the bit streamthan voice.

The present invention, however, is intended for use with a voicechannel, with voice bits on it. However, as noted above, the voice bitsare less robustly encoded, therefore, they are more prone to errors, andthe BER estimation on the user channel is not as accurate. However, BERestimation on the user channel is sufficient for continuous monitoringof the BER. Therefore, the present invention utilizes BER estimation onthe user channel to monitor channel conditions until a more accurateestimation is required.

FIG. 4 is a functional block diagram illustrating the functionsperformed in the method of the present invention when estimating the BERfor a speech transmission on the user channel. The 260 bits are receivedat a receiver detector 41 and are passed to a de-interleaver 42 whichseparates class 1 bits and class 2 bits. When Class 1 bits are received,a 1/2 decoder 43 decodes them into 77 bits. Of those 77 Class 1 bits,only 12 Class 1a bits have the CRC error detection scheme applied tothem. Thus, out of 159 voice bits (77 class 1 bits and 82 class 2 bits),the accuracy of only 12 bits is known. The decoded class 1a bits and theCRC are sent to a CRC decoder 44. If the CRC applied to those 12 Class1a bits is good, then the frame is good. If there is a CRC discrepancy,then the frame is dropped by the CRC decoder 44 as a frame error. Theframe erasure rate (FER) is measured over 26 frames, and is the ratio ofthe frames that did not pass the CRC check to the total number offrames.

The CRC process goes in parallel with the BER estimation process. BER iscalculated on every received frame, whether the frame is rejected by theCRC decoder 44 or not. In other words, BER is calculated on all framesthat were decoded--in the frames where the errors were all corrected bythe system, and also in the frames where all the errors were notcorrected by the system. In those frames where the errors are notcorrected by the system, the bits are unreliable, and the BER that iscalculated for those frames is not reliable.

The decoder removes the redundancy bits and produces a corrected bitstream which is sent to a 1/2 encoder 45 identical to the encoderutilized at the transmitter (not shown). The output, which approximatesthe bits transmitted, are passed to a BER detector 46 where they arecompared with the detected class 1 bits. Class 2 bits are passeddirectly from the de-interleaver 42 to a speech decoder 47. Decodedclass 1a bits and class 1b bits are also passed to the speech decoder 47from the 1/2 decoder 43.

The residual BER is the BER calculated on only the frames that pass theCRC check. The present invention monitors the BER and the residual BER.Monitoring the residual BER and the frame erasure rate (FER) provides abetter, more reliable estimate of the actual BER. The system monitorsthe residual BER in parallel with the FER because residual BER onlyindicates the BER on the accepted frames. It does not indicate how manyframes were refused.

When channel conditions are good, the system makes an accurate BERestimation to determine whether the BER is at BER_(up) 16 (FIG. 2). Thisis done by interrupting the voice on the user channel long enough tosend a short FACCH message over the FACCH channel and estimate the BER.The FACCH message replaces voice for one or two frames while the systemmakes an accurate BER estimate. Then a reliable decision can be madewhether to change to high level modulation. The FACCH message is sent onthe downlink (i.e., from the base station to the mobile station) andorders the mobile station to report the number of bit errors in theFACCH message it received. The mobile station replies by sending a FACCHmessage containing the requested information (i.e., bit errors on theFACCH downlink) (BE_(F).sbsb.--_(DL)). The base station receives themessage from the mobile station and determines the bit errors on theFACCH uplink (BE_(F).sbsb.--_(UL)).

FIG. 5 is a functional block diagram illustrating the functionsperformed in the method of the present invention when estimating the BERduring transmission of a Fast Associated Control Channel (FACCH) messageon the FACCH channel. The 260 bits are received at a receiver detector51. They are sent to a 1/4 decoder 52 which is matched to the encoderthat was used at the transmitter (not shown). The output of the decoder52 is a 16 bit CRC and 49 bits of data. A CRC decoder 53 checks the CRC,which is the error detection mechanism, and ascertains whether the CRCstill matches the 49 bits, and whether there are errors left in thedecoded bits. If there are errors detected, there is a frame error andthe CRC decoder discards the frame because the coding/decoding schemedid not manage to prevent errors. In parallel with this, the 49 decodedbits and the 16 CRC bits are sent to a 1/4 encoder 54 identical to theone that was used in the transmitter, in order to regenerate the 260transmitted bits. The regenerated bits and the detected bits are sent toa BER detector 55. By comparing the 260 bits that were detected and the260 bits that were regenerated, the BER detector determines the BER.Thus, the BER comes from the BER detector 55, and the frame error rateor frame erasures come from the CRC decoder 53.

FIGS. 6A-6C are a flow chart illustrating the steps of the bit errorrate estimation process of the preferred embodiment of the presentinvention. In general, there are three levels in the estimation process.On level 1, the process monitors the BER uplink and downlink on thevoice channel with the existing method of estimating BER. When the BERuplink and downlink are estimated to be lower than predeterminedthresholds for a specified period of time, then the process moves tolevel 2. On level 2, the process measures residual BER and the frameerasure rate (FER) on the uplink, and if these measurements indicatethat there are no errors in the system, then frames are not dropped. Theprocess then moves to level 3. On level 3, a FACCH message replaces thevoice and is sent over the downlink FACCH channel. An accurate estimateis then made of the number of bit errors on the downlink FACCH channel.The bit errors are then converted to a BER, and the downlink BER isreported by the mobile station in a FACCH uplink message on which thesystem evaluates the uplink BER.

Referring to FIG. 6A, the level 1 process monitors the BER uplink anddownlink on the voice channel with the existing method of estimatingBER. The process begins at step 61 where a counter C₁ is reset to zero(0). Counter C₁ counts the number of times that the procedure fails tomeasure a low BER at steps 66, 74, and 84. Counter C₁ counts up toC₁.sbsb.--_(MAX), which may be set, for example, at a value of 2. Atstep 62, it is determined whether or not C₁ ≦C₁.sbsb.--_(MAX). If C₁ isnot less than or equal to C₁.sbsb.--_(MAX), then the process moves tostep 63 where the process exits and reports a failure. The failure maybe reported by a code indicating that an accurate low BER value couldnot be determined. If, however, it is determined at step 62 that C₁ isless than or equal to C₁.sbsb.--_(MAX), then the process moves to step64 where a timer T₁ is reset to zero (0). Timer T₁ measures the timeperiod over which the BER is monitored in level 1, and may be set toapproximately 5 seconds.

The process then moves to step 65 where the process monitors the BER onthe uplink (BER_(UL)) and the BER on the downlink (BER_(DL)). At step66, it is determined whether or not BER_(UL) <B_(UL).sbsb.--₁, andBER_(DL) <B_(DL).sbsb.--₁. B_(UL).sbsb.--₁ and B_(DL).sbsb.--₁ arepredetermined thresholds for BER on the uplink and downlink,respectively. If not, the process moves to step 67 where the counter C₁is incremented. The process then returns to step 62. If yes at step 66,the process moves to step 68 where it is determined whether or not timerT₁ has expired. If not, the process returns to step 65 and continues tomonitor BER_(UL) and BER_(DL). If, however, timer T₁ has expired, theprocess moves to step 71 in FIG. 6B.

Referring to FIG. 6B, the level 2 process measures residual BER (RBER)and the frame erasure rate (FER) on the uplink. At step 71, a timer T₂is reset. Timer T₂ measures the time period over which the RBER and theFER are monitored in level 2, and may be set to approximately 5 seconds.At step 72, the RBER and the FER measurements are reset, and the processthen monitors the BER on the downlink, and the RBER and FER on theuplink (RBER_(UL) and FER_(UL)) at 73. The process then moves to step 74where each of these measurements are compared to various thresholds. Itis determined whether or not the BER_(DL) <B_(DL).sbsb.--₂ ; RDER_(UL)<B_(UL).sbsb.--₂ ; and FER_(UL) =0. B_(DL).sbsb.--₂ and B_(UL).sbsb.--₂are predetermined thresholds for BER on the uplink and downlink,respectively. If not, the process moves to step 75 where the counter C₁is incremented. The process then moves to step 76 where the processreturns to step 62 in FIG. 6A. If yes at step 74, the process moves tostep 77 where it is determined whether or not timer T₂ has expired. Ifnot, the process returns to step 73 and continues to monitor BER_(DL),RBER_(UL), and FER_(UL). If, however, timer T₂ has expired, the processmoves to step 81 in FIG. 6C.

Referring to FIG. 6C, the level 3 process makes an accurate estimate ofthe number of bit errors on the FACCH channel, converts the bit errorsto a BER, and reports the BER by the mobile station. At step 81, a FACCHcounter C_(F) is reset to zero (0). The FACCH counter C_(F) counts thenumber of times that FACCH messages are sent in level 3, up to a maximumpredetermined limit of C_(F).sbsb.--_(MAX). At 82, a timer T₃ is resetto zero (0). Timer T₃ measures the time interval between FACCH messagesin level 3, and may be set to expire at approximately 2 seconds. Theprocess then moves to step 83 and monitors the BER_(DL), RBER_(UL), andFER_(UL), in other words, the BER on the downlink, and the RBER and FERon the uplink. The process then moves to step 84 where each of thesemeasurements are compared to various thresholds. It is determinedwhether or not the BER_(DL) <B_(DL).sbsb.--₃ ; RBER_(UL)<B_(UL).sbsb.--3 ; and FER_(UL) =0. If not, the process moves to step 85where counter C₁ is incremented. The process then moves to step 86 whereit returns to step 62 in FIG. 6A. If yes at step 84, the process movesto step 87 where it is determined whether or not timer T₃ has expired.If not, the process returns to step 83 and continues to monitorBER_(DL), RBER_(UL), and FER_(UL). If, however, timer T₃ has expired,the process moves to step 88.

At step 88, the process sends and receives a FACCH message on the FACCHchannel and makes an accurate estimate of the bit errors on the FACCH(BE_(F)) on the uplink and the downlink. The base station sends amessage telling the mobile station to report the number of bit errors inthe last FACCH message it received. The mobile station replies bysending a FACCH message containing the requested information (i.e., biterrors on the FACCH downlink) (BE_(F).sbsb.--_(DL)). The base stationreceives the message from the mobile station and determines the biterrors on the FACCH uplink (BE_(F).sbsb.--_(UL)). At step 89, theprocess records the largest BE_(F).sbsb.--_(UL) and the largestBE_(F).sbsb.--DL to date.

The process then moves to step 90 and increments the FACCH counterC_(F). At 91, the process determines whether or not the number of timesthat FACCH messages have been sent in level 3 is greater than themaximum predetermined limit of C_(F).sbsb.--MAX. If not, the processreturns to step 82. If, however, C_(F) >C_(F).sbsb.--_(MAX), the processmoves to step 92 and exits and reports the largest BE_(F).sbsb.--_(UL)and the largest BE_(F).sbsb.--_(DL) to date.

It is thus believed that the operation and construction of the presentinvention will be apparent from the foregoing description. While themethod, apparatus and system shown and described has been characterizedas being preferred, it will be readily apparent that various changes andmodifications could be made therein without departing from the spiritand scope of the invention as defined in the following claims.

What is claimed is:
 1. A method of estimating uplink and downlink biterror rates (BER) in a radio telecommunications network, said methodcomprising the steps of:monitoring the uplink and downlink BER on avoice channel; determining that said uplink and downlink BER are lowerthan a first set of predetermined thresholds for a specified period oftime; measuring residual BER on the voice channel uplink; measuring theframe erasure rate (FER) on the voice channel uplink; determining thatthe downlink BER, the uplink residual BER, and the uplink FER are lowerthan a second set of predetermined thresholds for a specified period oftime; sending a Fast Associated Control Channel (FACCH) message on theFACCH channel downlink; estimating the number of bit errors on the FACCHchannel downlink; converting the number of bit errors to a downlink BER;reporting the downlink BER in a message on the FACCH channel uplink; andestimating the uplink BER from the message received on the FACCH channeluplink.